Hydreigon is an odd dragon. Being the pseudo-legendary Dragon type of Gen V, it has more in common with Latios than it does Dragonite or Garchomp. Indeed, it's main competition for a slot on the team is often Latios himself, as Latios has higher Special Attack, Special Defense, and Speed, but is often hindered by it's Psychic subtyping. Hydreigon trades Latios's weaknesses to Ghost and Dark for a weakness to Fighting instead, and access to a handful of moves that Latios doesn't. It has a good Special movepool and an acceptable Physical and support movepool, however it's main niche is a Special attacker/sweeper.

It's one of the few dragons that are legal in a VGC2011-style team, such as this year's Autumn Friendly WiFi Tournament. For that reason, it is often seen on those style teams, especially when such a tournament is approaching.

Gen V Level-up Moves

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Hydreigon's standard doubles setup, seen most of the time on GBU. It's designed to take advantage of it's dual STABs. Draco Meteor/Dragon Pulse OHKOs most dragons with Life Orb/Dragon Gem unless they invest heavily in Special Defense or carry a Haban Berry, and does huge neutral damage to anything that doesn't resist it. STAB Dark Pulse greatly hurts Psychics and Ghosts. Fire Blast/Flamethrower destroys Ferrothorn (which otherwise walls most other dragons), Scizor, and inflicts significant damage to Steels not named Heatran, most Grass types, and most Bugs not named Volcarona. Protect is self explanatory for Doubles and Triples, but a fourth attack could be used if desired, or U-turn.

A Timid nature lets you outspeed Jolly Haxorus and speed tie with opposing Timid Hydreigon, while the power boost from a Modest nature is noticeable. Max speed EVs is needed with a Modest nature to outspeed neutral Haxorus. Besides Haxorus, you don't outspeed anything significant.

Additional Comments:
What sets this set apart from Latios is the ability to run Fire Blast/Flamethrower instead of the weaker HP Fire, and STAB Dark Pulse to inflict harm to Psychic and Ghost types instead of Shadow Ball which Latios must use to achieve the same coverage. In Triples, Dragon Pulse and Dark Pulse have the advantage of hitting any Pokemon on the field regardless of position, whereas Draco Meteor is limited to adjacent targets only.

Life Orb is there to provide an all-around attack boost, while Dragon Gem is there for a one-time boost to Draco Meteor / Dragon Pulse, and often ensure a OHKO of most dragons through even a Haban Berry. A Chople Berry or Haban Berry could be used for a one-time resistance to Fighting or Dragon moves, but Latios can OHKO with a Dragon Gem even through a Haban Berry.

Hydreigon can utilize Choice Scarf to great effect and net a surprise kill to opposing dragons that are expecting a standard Life Orb / Dragon Gem Hydreigon. A Timid nature is recommended to outspeed Jolly Choice Scarfed Haxorus, but even with a Modest nature Hydreigon reaches 225 speed, enough to outspeed almost anything without a speed boost or Choice Scarf of their own.

Draco Meteor and Fire Blast/Flamethrower are the main moves on this set, while the remaining two moves can be tailored to your liking.

An alternative EV spread is 140 HP / 252 SpA / 116 Speed with a Timid nature. This provides some moderate bulk while still maintaining enough speed to do it's job.

This set is certainly one of Hydreigon's most odd setups, but is fantastic for catching slower status-inducing Pokemon off guard, especially the likes of Amoonguss. If played right it provides Hydreigon a much needed defensive buffer, since few things can hit through a Substitute. It allows Hydreigon to stall out a Close Combat user like Terrakion until it's defenses are low enough from repeated Close Combats to net a KO. It can also be used to help stall out a Trick Room team.

Flamethrower is recommended over Dark Pulse since Steel types are very abundant in GBU.

Other Options:
Hydreigon has access to Earth Power, Focus Blast, Surf, and Charge Beam from the Special side. On the Physical side, it has access to Crunch, Dragon Rush/Outrage, Earthquake, Rock Smash, Acrobatics and Aerial Ace, as well as the elemental fangs, but it's generally outclassed by other dragons on the Physical side, and generally outclassed by Salamence when going mixed. Hydreigon also has access to support moves like Thunder Wave, Toxic and Taunt, but other Pokemon are generally better at this kind of support role. On the other hand, a support Hydreigon is rarely seen and can be a nasty surprise for the unprepared.

Hydreigon can also use Choice Specs or Choice Band, but again it's generally outclassed by other dragons. Caution also needs to be used if you're using Draco Meteor on a choice set, as the Special Attack drop is severe.

Because of it's awkward speed, Hydreigon can actually function surprisingly well in a Trick Room team, but it needs a Macho Brace to "outspeed" most of it's common threats in Trick Room. However, this comes at a cost of survivability or power if you opt not to use Leftovers or Life Orb.

Partners:
Anything that can provide Tailwind or Paralysis support will do wonders to help Hydreigon gain a speed advantage. Something to take down Fighting types is appreciated, as Hydreigon has few options to hit them super effective. Hydreigon works very well on a Sun or Rain team, depending if it's using Flamethrower/Fire Blast or Surf, and indeed many partners don't mind getting hit by Surf. Intimidate support is greatly appreciated against opposing Fighting types, as it allows Hydreigon to survive attacks longer. Thanks to Hydreigon's Levitate, it's an excellent partner with most Earthquake users.

Counters:
Latios and Terrakion are lethal enemies, and can dispatch Hydreigon before it can attack due to their much higher speed and STAB Dragon and Fighting attacks, respectively, even through resist berries. If running Modest, Hydreigon is outsped by Jolly Haxorus and opposing Timid Hydreigons. Rain teams reduce Flamethrower/Fire Blast's effectiveness in half. Fighting types like Conkeldurr and Mienshao can often inflict massive damage without worry. Tyranitar can shrug off most special attacks in sand besides Focus Blast. Volcarona outspeeds and can OHKO with Bug Buzz. Scizor can take almost any neutral/resisted attack and OHKO back with Bug Bite or Superpower. Ferrothorn completely walls without Flamethrower/Fire Blast or Focus Blast. Eviolite Chansey and Blissey shrugs off everything except Focus Blast on a special set, but can't take repeated physical attacks.

Hydreigon isn't so much a Pokemon that I use often, but it is one that I see an awful lot of in doubles. 9 times out of 10 it's using the standard set listed, making it fairly predictable and easily dispatched.

The U-turn was a bit random, but after a Draco Meteor it was sometimes nice to be able to break Focus Sash's or deal a little extra damage while switching.

I've been doing a few Triple battles recently, and Dragon Pulse is probably the preferred move here over Draco Meteor; as it is able to hit all three of the opponents regardless of Hydreigon's position (as all "Pulse" moves can, which gives Hydreigon some extra value with both Dragon and Dark Pulse.)

Your forgetting choice scarf variants. Oh that thing is so unexpected(beat me in t8 this yr, and many 1800+ jp players use it, and its pretty damn annoying). Aside from that the only reason you would wanna choose hydrei over lati, is his typing and access to fire blast. So yea scarf hydrei is mainly annoying, because lets say this:

Turn 1
Hydrei+X
latios/garchomp/salamence/latias+X

All faster dragons. They think they have the upperhand, and then Draco meteor, for ze ohko.

At Worlds 2011, both BlueCookies and I ran Substitute Hydreigons to great success. In combination with its nice resistances and team support, Hydreigon has plenty of opportunities to set one up, with some nice benefits:

- Status protection from Amoonguss' Spore
- If you predict correctly, you can set up a defensive buffer while your opponent wastes a turn Protecting. With Substitute in place, your opponent can no longer prevent Hydreigon from moving by virtue of KOing it first, for the most part.
- Greater ability to stall out Trick Room
- Allowing Hydreigon to stall out a Close Combat user until their defenses drop low enough for a KO

I ran Timid with max SpA\Speed, he ran something slower and bulkier with T-Wave support. This would probably be an ideal GBU\2012 set: